In recent years there has been increasing use of hot-dip zinc coated and galvannealed steel sheet in automotive body panels, and other related structures. A cold-rolled steel strip can be given a good formability by means of a heat treatment such as that disclosed in U.S. Pat. No. 4,361,448 (incorporated herein by reference). In this process, after annealing at a temperature T.sub.1 (720.degree. to 850.degree. C.) the steel strip is slowly cooled to a temperature T.sub.2 (600.degree. to 650.degree. C.). At this point the steel is rapidly quenched in a zinc bath to a temperature T.sub.3. The time interval for revealing the temperature between T.sub.2 and T.sub.3 is about 0.5 seconds.
In the arrangement of the U.S. Pat. No. 4,361,448 a zinc bath and a zinc pump, with nozzles, are used. Molten metal having the same temperature as the zinc bath is pumped through a spout to the immersion point of the steel strip. As a result the end temperature T.sub.3 of the rapid cooling process is rather high, and the steel strip does not reach the temperature of the zinc bath during the entire immersion time (about two seconds).
A steel strip traveling through a zinc bath causes a laminar zinc flow following the surface of the steel strip. The heat from inside the steel strip raises the temperature of the laminar zinc flow (layer) to a value higher than the operating temperature of the zinc bath. Iron and zinc react strongly in a conventional zinc bath (containing 0.15 to 0.25% aluminum) at temperature above 480.degree. C. This results in a thick intermetallic layer formed on the zinc coating.
In order to achieve a good formability of the zinc coating, the intermetalic layer should be as thin as possible. In the method disclosed in U.S. Pat. No. 4,971,842 (incorporated herein by reference), the thickness of the intermetallic layer is controlled by rapidly cooling the steel product. This is accomplished by quenching the steel in a bath of molten zinc, and controlling the structure of the coating to be formed on the steel product in the quenching by directing a flow of molten zinc, cooled to a temperature below the operating temperature of the zinc bath, toward the steel product as it moves through the zinc bath.
Preferably the first flow of molten zinc is directed towards the steel product close to the immersion point thereof and obliquely to the movement direction of the steel product by means of a set of first nozzles. A second flow of cooled molten zinc is directed essentially perpendicularly toward the steel product at a point after said obliquely directed flow, by means of a second set of nozzles.
The flow of molten zinc directed towards the steel product is cooled by means of a heat exchanger cooler, preferably to a temperature 1.degree. to 15.degree. C. below the operating temperature of the zinc bath. The flow of zinc through the cooler to the nozzles is kept separate from the rest of the zinc bath. The essential feature of locally cooling the zinc bath is the additional important advantage that the iron content of the zinc bath is lowered.
The iron content of a zinc bath used, in a continuous hotdip galvanizing process of thin steel sheet is generally at the saturation point. Even a small change in the temperature causes a precipitation of iron and zinc. This occurs either at the bottom of the bath or as a drift of precipitates onto the surface of the steel strip to be galvanized, which impairs the quality of the coating.
Thus, to maintain a good quality, variations in the temperature of the zinc bath should be avoided. Therefore, some galvanizing lines are provided with separate pots for the preliminary melting of zinc so that the melting temperature of the zinc to be added would not change the temperature of the zinc bath.
The solubility of iron in molten zinc is generally a linear function of the temperature. At normal galvanizing temperature of approximately 455.degree. C., the iron content is about 0.040%, while at a temperature of about 440.degree. C. the iron content is about 0.015%. To improve the quality of a hot-dip galvanized thin steel sheet, dross, such as Fe.sup.- Zn precipitates (slag particles), on the zinc coating must be avoided. Thus, it is advantageous to lower the iron content in the zinc bath from a saturated state, so that use of different galvanizing temperatures is possible without precipitation of very small Fe--Al--Zn particles from the molten zinc. These particles are a combination of bottom dross (FeZn.sub.7) and top dross (Fe.sub.2 Al.sub.5). These particles are discussed in greater detail in the publication by Kato et al., entitled Dross Formation and Flow Phenomenon in Molten Zinc Bath, Galvatech '95 conference proceedings, Chicago, 1995, pages 801-806. This publication is incorporated herein by reference as background material elaborating upon the nature of the types of dross particles that are formed in the environment in which the present invention operates. When the zinc flows toward the steel strip, small Fe--Al--Zn particles adhere as an even layer to the surface of the steel product and leave the zinc bath as a part of the zinc coating.
To keep the Fe--Al--Zn particles as small as possible and homogeneously distributed, the temperature and the rate of the zinc flow should preferably be at constant value. The heat loss caused by the zinc cooler can be compensated by adjusting the speed of the steel product the temperature of which is higher than the temperature of the zinc bath.
A major problem with the operation disclosed in U.S. Pat. No. 4,971,842 is dross-pick up on the strip during the hot-dip coating process due to the suspended dross in the bath. The presence of dross particles of Fe--Zn and Fe--Al intermetallics within coating is of particular concern. First, stamping and forming operations can cause some "print-through" and other defects that show up in the painted appearance of the product. This is of particular concern when the steel is used in the automotive and appliance end-user areas. In particular, galvanized surface blemishes, attributable to dross particles, become highlighted when high gloss paint finishes are applied on them.
The dross particles can also cause operational problems when they build-up on the sink roll (element 4 in FIG. 1). This necessitates down-grading the steel product to less critical categories, and/or shutting the line down periodically to clean or change the affected roll results in lost production.
Even if perfect zinc bath chemistry management using conventional galvanizing technologies is conducted, dross crystallization is unavoidable due to aluminum addition, iron dissolution from the steel strip, insufficient temperature uniformity, and insufficient chemical bath homogeneity. The dross pick-up problem can theoretically be avoided only if the coating is performed with a dross free zinc bath composition.
While the system described in U.S. Pat. No. 4,971,842 has improved the temperature uniformity of the bath, chemical homogeneity has not been sufficiently improved. However, when the zinc flows towards the steel strip, small Fe--Al--Zn particles adhere as an even layer to the surface of the steel product and leave the zinc bath as part of the zinc coating. This is due to the insufficient performance of the second flow from a second set of nozzles towards the steel strip. Also, the flow pattern as shown in FIG. 1 is insufficient to provide chemical homogeneity of the zinc bath. This situation exists because the volume of the whole bath is insufficiently agitated throughout it's entirety thereby allowing some local accumulation of dross within the bath. Also, this and the conventional systems do not provide sufficient cleaning of the zinc roll (element 4 in FIG. 1). As a result, dross build-up on the roller surface cannot be prevented without a mechanical scrapper, which presents it's set of problems.
Thus, while the cooler described in the U.S. Pat. No. 4,971,842 does decrease the amount of dross particles in the zinc bath, it cannot provide perfectly dross free bath composition and dross free coating. The conventional art has also failed to adequately address the problem of dross control within hot-dipped galvanized processes, so that a cooler/cleaner system and process that can do so is very desirable.